a) Field of the Invention
The present invention relates to a zoom lens system, and more specifically a zoom lens system in particular for compact cameras.
b) Description of the Prior Art
As zoom lens systems for compact cameras, there are conventionally known zoom lens systems including one which consists of two lens units, i.e., a first positive lens unit and a second negative lens unit, and is configured to change a focal length by varying an airspace reserved between the two lens units, and another zoom lens system which consists of three lens units, i.e., a first positive lens unit, a second positive lens unit and a third negative lens unit, and is configured to change a focal length by varying airspaces reserved among the lens units.
Out of these zoom lens systems, the zoom lens system which consists of the two lens units permits not only simplifying a lens barrel structure and a drive mechanism but also reducing a number of lens components to be used, thereby being more advantageous for lowering a manufacturing cost than the zoom lens system which consists of the three lens units. On the other hand, the zoom lens system which consists of the three lens units is more excellent in that it can be configured to have a zoom ratio as high as possible and be compact as a whole.
Lens system disclosed by Japanese Patents Kokai Publication No. Hei 4-260016 and Kokai Publication No. Hei 5-188296 are known as conventional examples of zoom lens system which is configured to use lens elements in a minimum number to reduce a manufacturing cost. Out of these conventional examples, the former adopts a first positive lens unit which consists a positive lens component, a second lens unit which consists of a negative lens component and a positive lens component, and a third lens unit which consists of a negative lens component. The latter zoom lens system adopts a first lens unit which consists of a negative lens component and a positive lens component, a second lens unit which consists of a negative lens component and a positive lens component, and a third lens unit which consists of a negative lens component.
To configure the zoom lens system which consists of the first positive lens unit, the second positive lens unit and the third negative lens unit so as to have a higher zoom ratio and more compact design, it is sufficient to strengthen a refractive power of each of the lens components by shortening a radius of curvature on each lens surface. In such a case, however, aberrations are produced in larger amounts and remarkably varied during zooming. To reduce variations of aberrations during zooming so as to obtain aberrations favorably corrected in an entire zooming region and a sufficiently high vari-focal ratio, each of the lens units must be composed of a larger number of lens components and the zoom lens system is inevitably enlarged as a whole. Furthermore, it becomes more difficult to manufacture the zoom lens system at a low cost as it is composed of a larger number of lens components. When a refractive power of each of the lens component is weakened to reduce variations of aberrations during a change of a magnification, the lens units must be moved for longer distances for changing a magnification, thereby making it impossible to configure the zoom lens system compact.
Furthermore, the zoom lens system which consists of the three lens units described above produces positive spherical aberration and aggravates longitudinal chromatic aberration at a tele position since the third lens unit has a strong negative refractive power. Furthermore, the zoom lens system produces positive distortion in a larger amount and aggravates coma at a wide position.